The present invention relates to a piston for internal combustion engines, particularly a piston of the two-piece, or articulated, type.
The two-piece pistons represent a major contribution to an improvement in the performance of internal combustion engines. The basic construction of the two-piece piston comprises a top portion, or head, generally of cast iron or steel, from whose lower part and integral with it depend a pair of legs for bearing a wrist pin which connects the head portion to another member, the skirt, generally of aluminum or an aluminum alloy, designed to guide the piston in the cylinder liner.
The two-piece piston offers several advantages. One important advantage is a lower top land height, thus decreasing the crevice volume, i.e., the volume within the room defined by the top land and the cylinder liner, hereby reducing engine emissions. The piston head is made of materials having a low coefficient of thermal conductivity, generally iron or steel, which helps to maintain ideal temperatures for a complete combustion of the fuel-air mixture and, accordingly, a reduction of engine emissions. Also, the use of iron or steel for the piston top makes this component compatible with other engine components such as the cylinder liner and the cylinder head, also of iron or steel, regarding coefficients of thermal expansion.
There are also many other advantages provided by this type of piston, such as, for instance, the high strength of the head even at high temperatures, as a result of the material of which the head is usually made. The lower portion, or skirt, generally of aluminum or an aluminum alloy, is extremely light, thus contributing to reduce the total weight of the assembly.
Another advantage of the two-piece, or articulated, pistons is that their components are independent and do not make contact with each other with the piston in operation in the engine.
Thus, the heat generated in the combustion region is not transmitted from the head to the skirt, whereby it is possible to mount the piston in the cylinder liner with extremely small clearances thereby attaining a remarkable reduction in engine noise levels even at cold starts.
In conclusion, it is a common understanding that two-piece, or articulated, pistons are quite appropriate for internal combustion engines, particularly diesel engines having a high specific output and a low level of emissions and noise. Therefore, the articulated pistons meet the most advanced operating demands, including strict legal requirements concerning emissions and engine noise. While exhibiting an advanced concept, the two-piece pistons may be i proved in order to overcome certain operating shortcomings. One problem attending all two-piece pistons results from the thermomechanical load to which the pin bosses are subjected. When the piston is on its top dead center or close to it, the fuel-air mixture is burned and generates a heavy thermomechanical load which is transmitted to the pin bosses depending from the piston head and whose function is to accommodate the piston pin.
Owing to the load transmitted by the head to the pin bosses, the later tend to deform outwardly in the direction of the skirt innerwall, urging the skirt against the cylinder liner. These deformations subject the pin bosses to a structural fatigue, thus reducing their service life. In addition, these deformations impose a need for a substantially reinforced piston pin capable of withstanding the bending caused by the compressive forces arising out of such deformation. This known solution is not recommended as it contribute to an increase of the total weight of the assembly. There have been few alternatives for improving the strength of the piston assembly, and most of these alternatives cause a weight increase.